This chapter introduces the optical network. We begin with a survey of three generations of digital transport networks, followed by a discussion of the extraordinary capacity of optical fiber. The optical network marketplace is examined with a look at current and projected installations. Next, we examine the key nodes (machines) that make up the optical network, then we look inside a node to learn about its components. The chapter concludes with a general explanation of the attributes of optical fiber.

This chapter is from the book

This chapter is from the book

Three Generations of Digital Transport Networks

The focus of this book is on third generation digital transport networks,
usually shorted to 3G, or 3rd generation, transport networks. The main
characteristics of three generations of digital transport networks are provided
in Table 11. The information in this table will be helpful as you read the
remaining chapters in this book. Most of the terms in the table are
self-explanatory, or, if not, are explained in this chapter.

Table 11 Three Generations of Digital Transport (Carrier)
Networks

Name

Family

Designed for

Mux/SW Schemes at Inception

Principal Media at Inception

Capacity

Typical Payload

Protocol Inter-Working?

T1/E1

First

Voice, Non-BOD, Static

TDM/E/E/E

Copper: (Early1960s)

Mbit/s

Fixed Length

No

SONET/SDH

Second

Voice, Non-BOD, Static

TDM/O/E/O

Copper, Fiber: (Mid1980s)

Gbit/s

Fixed Length

Somewhat: PPP, IP, ATM

OTN

Third

Voice, Video, Data, Tailored QOS, BOD, Dynamic

WDM/O/O/O

Fiber (Late1990s to Early 2000s)

Tbit/s

Fixed or Variable Lengths

Yes: PPP, IP, ATM, MPLS

The first column in the table is the name (or names) usually
associated with the technology. The first generation systems are known as T1 or
E1. The second generation systems are called SONET (for the Synchronous Optical
Network) or SDH (for the Synchronous Digital Hierarchy). These terms are
explained in more detail in later parts of this book. However, the industry has
not yet settled on a handle for the third generation digital carrier network,
but the term Optical Transport Network (OTN) is widely used. The second column
identifies the generation family.

The third column shows what kinds of user payloads the networks are designed
to support. Although the first and second generation networks are designed to
support voice traffic, they can and do transport data and video images. But they
are not "optimized" for data and video traffic. In contrast, the 3G
transport network is designed to support voice, video, or data payloads. When
used with multiprotocol label switching (MPLS), the resource reservation
protocol (RSVP), and DiffServ, as well as some of the new specifications dealing
with optical bandwidth on demand, they are also designed to provide tailored
quality-of-service (QOS) features for individual customers. The point will be
made repeatedly in this book that the 3G transport network no longer consists of
fixed, static "pipes" of capacity; it can dynamically change to meet
the changing requirements of its users.

The third column also contains the notations of Non-BOD or BOD. The first and
second generation systems are not designed to provide bandwidth of demand (BOD).
The bandwidth is configured with crafting operations at each node. 3G systems
are more dynamic and allow bandwidth to be requested on demand.

The fourth column lists the predominant multiplexing schemes: TDM or WDM. The
fourth column also lists the manner in which the networks switch traffic when
they were first deployed (at their inception). First generation systems were
solely E/E/E operations: (a) they accepted electrical signals (the first E), (b)
processed them (the second E), and (c) sent them to another node (the third E).
Second generation systems are O/E/O operations: (a) they accept optical signals
(the first O), (b) convert them to electrical signals for processing (the E),
and (c) convert the electrical signals back to optical signals for transmission
(the second O). Third generation systems are intended to be all optical (O/O/O),
in that they process optical payloads, and do not need to convert the bits to
electrical images for processing. Today, all three generations are mainly O/E/O
oriented.

The fifth column lists the principal media used by the technologies at their
inception, as well as the time that these networks were first introduced into
the industry. All three generations now use a combination of copper, fiber, and
wireless media.

The sixth column lists the typical capacity of the generation. It is evident
that each succeeding family has increased its transport capacity by orders of
magnitude.

The seventh column goes hand-in-hand with the third column ("Designed
For"). The first and second generation networks were designed for
fixed-length voice traffic, based on the 64 kbit/s payload, with a 125-μsec
clocking increment. The third generation network supports this signal, but also
supports variable-length payloads, an important capability for carrying data
traffic. As well, the first and second generation networks can carry
variable-length traffic, but they are not very efficient in how they go about
transporting variable-length data traffic.

The eighth column explains whether any of the generations were designed to
interwork with and directly support other protocols. T1/E1 was not so designed;
again, 1st generation transport systems were set up to support voice traffic.
Any efforts to devise methods of carrying other payloads were an afterthought
and in vendor-specific procedures. With the advent of 2nd generation systems
with SONET/SDH, efforts were made by the standards groups to define procedures
for carrying certain kinds of data traffic, and many manufacturers adapted these
standards into their products.

3rd generation transport networks are geared toward supporting many kinds of
payloads, and specifically the Internet, ATM, and MPLS protocol suites. As we
shall see as we proceed though this book, extensive research has resulted in
many specifications defining how MPLS contributes to the operations of the third
generation digital (optical) transport network.

All Features Are Not Yet Available

Not all the features and attributes cited in Table 11 are available in
3G transport networks. In fact, third generation transport networks are just now
appearing in the marketplace, and some capabilities that are touted for them are
still in the lab. Nonetheless, many people think full-featured 3rd generation
transport networks will be in the marketplace by around 2004. Certainly, pieces
are emerging, such as bandwidth on demand, and of course, WDM and terabit
networks. Other parts of 3G transport networks have yet to be implemented. For
example, O/O/O operations are far from reaching commercial deployment on a mass
scale.

Optical Fiber Capacity

To gain an appreciation of the transmission capacity of optical systems
operating today, consider the facts in Table 12. Prior to the advent of
optical fiber systems, a high-capacity network was capable of operating (sending
and receiving traffic) at several million bits per second (Mbit/s). These
electrical/electromagnetic transmissions take place over some form of metallic
medium such as copper wire or coaxial cable, or over wireless systems such as
microwave. In contrast, optical fiber systems transmit light signals through a
glass or plastic medium. These systems are many orders of magnitude
"faster" than their predecessors, with the capability of operating in
the terabits-per-second (Tbit/s) range.

Table 12 Magnitudes and Meanings

Magnitude

Term

Initial

Meaning

1 000 000 000 000 000 000=1018

exa

E

Quintillion

1 000 000 000 000 000=1015

peta

P

Quadrillion

1 000 000 000 000=1012

tera

T

Trillion

1 000 000 000=109

giga

G

Billion

1 000 000=106

mega

M

Million

1 000=103

kilo

k

Thousand

100=102

hecto

h

Hundred

10=101

deka

da

Ten

As depicted in Figure 11, a terabit fiber carries 1012
bits per second. At this rate, the fiber can transport just over 35 million data
connections at 28.8 kbit/s, or about 17 million digital voice channels, or just
under 500,000 compressed TV channels (or combinations of these channels). Even
the seasoned telecommunications professional pauses when thinking about the
extraordinary capacity of optical fiber.

A logical question for a newcomer to optical networks is, why are they of
much greater capacity than, say, a network built on copper wire, or coaxial
cable? The answer is that optical signals used in optical networks operate in a
very high position and range of the frequency spectrum, many orders of magnitude
higher than electromagnetic signals. Thus, the use of the higher frequencies
permits the sending of many more user payloads (voice, video, and data) onto the
fiber medium.

Figure 12 shows the progress made in the transmission capacity of
optical fiber technology since 1982 [CHRA99]. The top line represents
experimental systems, and the bottom line represents commercial systems. The
commercial results have lagged behind the experimental results by about six
years. The dramatic growth in the experimental capacity was due to improved
laboratory techniques and the progress made in dispersion management, a subject
discussed later in this book. As the figure shows, the transmission capacity of
optical fiber has been growing at an extraordinary rate since the inception of
the technology.